Injection devices (i.e. devices capable of metering and delivering a number of doses from a medication container) typically fall into two categories—manual devices and autoinjectors.
In a manual device—the user must provide the mechanical energy to drive the fluid through the needle. This is typically done by some form of button/plunger that has to be continuously pressed by the user during the injection. There are numerous disadvantages to the user from this approach. If the user stops pressing the button/plunger then the injection will also stop. This means that the user can deliver an underdose if the device is not used properly (i.e. the plunger is not fully pressed to its end position). Injection forces may be too high for the user—particular if the patient is elderly or has dexterity problems. The extension of the button/plunger is too great. This can be uncomfortable for the user to reach a fully extended button. The combination of injection force and button extension can cause trembling/shaking of the hand which in turn increases discomfort as the inserted needle moves.
Autoinjector devices aim to make self-administration of injected therapies easier for patients. Current therapies delivered by means of self-administered injections include drugs for diabetes (both insulin and newer GLP-1 class drugs), migraine, hormone therapies, anticoagulants etc.
Autoinjectors require some form of energy input in order to operate. Typically this is achieved by the user performing a “priming” or “cocking” action prior to the injection. This may make the device more complicated to use, as it increases the number of user steps. Also, the actions required, for example pulling, pushing or twisting to charge a spring, may be difficult for a user to perform, particularly if the user is elderly or has dexterity problems.
In an autoinjector device—the injection of the fluid is automatic once the device has been triggered. This overcomes many of the disadvantages of manual devices. Injection forces/button extension, hand-shaking and the likelihood of delivering an incomplete dose are reduced. Triggering may be performed by numerous means, for example a trigger button or the action of the needle reaching its injection depth. In some devices the energy to deliver the fluid is typically provided by a spring which must be recharged by the user in between doses. These devices also have a number of disadvantages. Performing the “reset” action increases the number of user steps (although this can sometimes be “concealed” by making it part of the dose-selection steps). The forces required to perform the reset step may be difficult to apply—particularly for elderly or those with dexterity problems. Depending on the device the reset force may be applied as a twisting action (torque) or a push/pull.
In WO 02/47746 A1 a device for auto-injection of a dose of medicament is disclosed, comprising a housing arranged to contain a medicament container therein and comprising a contact part intended to be applied against an injection site, a needle cover surrounding a needle arranged to the medicament container and extending at least the length of the needle, spring means capable of, upon activation, pushing the needle past the end of the needle cover as well as operating said medicament container to supply the dose of medicament, first locking means capable of locking said spring means in a pressurised state, first activating means capable of, upon manual operation, releasing said spring means for injection, characterised by a second locking means capable of locking said first activating means and a second activating means, capable of releasing said second locking means when said contact part is exposed to pressure.
The spring means used in this autoinjector is a helical spring. With this spring the force from the spring is proportional to the extension of the spring. Therefore, as the spring extends and energy is released the force decreases. Multiple-dose autoinjector mechanisms using these springs must therefore be designed to be capable of delivering the final dose in the spring's extended condition. However, with the spring compressed ready to deliver the first dose the force applied may be far in excess of what is necessary. This has two consequences. Firstly, the dose delivery is variable throughout the life of the device (earlier doses are delivered with greater injection speed) and secondly some of the energy stored in the spring is “wasted” as the spring applies excessive force for many of the doses. This means that the springs may be larger than necessary and the device will have to be engineered to cope with the increased forces. The same problem applies to helical springs applying either torque or linear force or for torsional springs applying torque.
WO 2006/130098 A1 discloses a device for delivery of predetermined doses of liquid medicament. The device comprises a servo drive spring acting in the way of a clock spring for generating a torque. The torque is used for rotating a drum which advances a threaded plunger rod. The rotation of the drum may be blocked by a pin engaging with a slot when a dose of medicament has been delivered.
U.S. Pat. No. 6,972,007 B2 discloses a device for administering an injectable product in doses, the device comprising a dosing member connected to a locking means, which may be held in latching positions thus taking off a spring force onto a driven member which serves for advancing a piston and consequently for delivering a set dose of medicament. The locking means is held in the locking position by grooves engaging with corresponding protrusions.
WO 2006/045529 A1 discloses an injection device having a helical spring adapted to provide a force in the axial direction of the injection device for ejecting a dose of medicament. A rotary movement of a dose indicator barrel is caused by the force of the resilient member acting on a thread. The rotational movement may be manually blocked by appropriately switching a locking member between a locking state and an unlocking state.
WO 2007/063342 A1 discloses a pen-type injector for receiving a medication container. The injector comprises a housing and a torsion spring coupled to a drive member. A dose setting knob is coupled to the spring and rotatably coupled to a housing such that rotation of the knob in a first direction results in compression or twisting of the spring. A user actuable button is coupled to the housing for axial motion relative thereto, the button being coupled to the torsion spring to unwind or expand in discrete steps with each press of the button, which is achieved by engaging teeth of sprung legs of a clutch collet with a toothed rack.